Fusible protective devices for cable protection



y 5, 1964 P. c. JACOBS, JR 3,132,223

FUSIBLE PROTECTIVE DEVICES FOR CABLE PROTECTION Filed June 19, 1961 2Sheets-Sheet l TEMPERATURE INVENTOR.

PHILIP G- JAGOBS JR BYNAMWW M 3,132,223 CABLE PROTECTION 2 Sheets-Sheet2 P. C. JACOBS, JR

May 5, 1964 FUSIBLE PROTECTIVE DEVICES FOR Filed June 19, 1961 INVENTOR.PHILIP C. JACOBS, JR BY MW W United States Patent r 3,132,223 FUSIBLEPROTECTIVE DEVICES FOR 1 CABLE PROTECTION Philip C. Jacobs, Jr.,Newtonyille, Mass, assignor to The Chase-Shawmut Company, Newburyport,Mass. Filed June 19, 1961, Ser. No. 118,048

7 Claims. (Cl. 200-120) have substantially the same degree ofinterrupting capacity 1 r and selectivity normally only encountered incurrentlimiting fuses.

A current-limiting fuse of standard design is generally not suitable foruse in cable networks.

It'is, therefore, another object of this invention to provide devices inthe nature of a current-limiting fuse adapted for service in cablenetworks.

It is possibleto distinguish between two general classes offusibleprotective devices for the protection of cable networks. In devices ofone class the fusible element is formed by a relatively large mass ofmetal having a relatively low fusing point. These devices are selectivebecause of the great time lag involved in their operation. The maindisadvantage of this type of protective devices is their relativelylimited interrupting capacity. In addition thereto this type of devicesinvolves relatively high watt losses. Devices of. the other abovereferred-toclasses include multiperforated ribbon-type fuse links of ametal such as silver, or-copper, having a high conductivity and a highfusing point and they further include a pulverulent arc-quenching filleras, for instance, quartz sand.

- This invention is concerned with the second type of fusible protectivedevices. Whilethis invention proposes to apply some of the principlesunderlying the design of a current-limiting fuseit represents a dnasticdeparture rfirom prior art devices for the protection of cable networksdesigned along lines similar to a current-limiting fuse.

United States Patent 2,770,757 to William S. Edsall, November 13, 1956,.Arrangement for the Protection of Cables Against Short-Circuit Currentsand Against Overloading is concerned with a fusible device for theprotection of cable networks designed along lines similar to thoseunderlying the design of a current-limiting fuse.

The above Edsall device is a composite device of which one unit producesa circuit interrupting break on the occurrence of shortecircuit currentsand another unit produces a circuit intenupting brea'k on theoccurrence. of overloads. I

It is, therefore, another objectof this invention to providesimplified'fusible devices for the protection of cable networks designedalong lines similar to those underlying the design of a current-limitingfuse but having one single integral unit for interrupting both.short-circuit currents and. overload currents. f

The above Edsall device is predicated on the concept that the "heatgenerated in the protective device should be minimizedand that the heatrequired for its operation shouldm ainly be furnished by the cableforthe protection of which it is provided, and the above Edsall device isfurther predicated on the concept that the time-current characteristicof the protectivedevice should follow as closelywas possible the smokingpoint characteristic, or the insulation damage characteristic, of thecable.

3,132,223 Patented May 5, l 964 ice concepts as far as the source ofheat for operating the device is concerned, but deviates drasticallyfrom the Edsall concept of t etching the characteristic of theprotective device with that of the cable. As long as it is the principalobject of a cable protective device to extend the life of cables as muchas reasonably possible, the characteristic of the protective deviceshould necessarily match as close as possible the insulation damagecharacteristic of the cable. Considerations of continuity of servicemay, however, make it desirable to more or less drastically depart fromthe above Edsall doctrine of matching of characteristics.

It is, therefore, a further object of this invention to provide fusibleprotective devices for the protection of cable networks which aredesigned along lines similar to a current-limiting fuse but areadaptedto maintain continuity of service even at the expense of the lifeof cables as long as there is no immediate explosion or fire hazard.

These and other objects of the invention and advantages thereof willbecome more apparent from the accompanyequation ing drawings and thefollowing description of a pre ferred embodiment of the invention.

In the drawings FIG. 1 is a diagrammatic representation of a portion ofa cable network;

FIG. 2 shows diagrammatically or qualitatively the temperaturedistribution along a fuse structure which is inserted into aconventionalfuse holder;

FIG, 3 shows diagnammatically or qualitatively the temperaturedistribution along a fusible protective device which isdesignedaccording to this invention;

FIG. 4 is in part a longitudinal section and in part a side elevation ofa protective device embodying this invention; and

FIG. Sshows the same device as FIG 4 taken along Referring now to thedrawings, and more particularly to FIG. 1 thereof showing a portion of acable network, it has been assumed-that a short-circuit has occurred atthe point S causing fault currents 1,, I and I to flow in the soundcables of the networl. If continuity of service is to be maintained thefuse 4 must blow to disconnect the short-circuited cable from thenetwork, while fuses 1, 2 and.3 must remain intact. In other words,fuses 1, 2, 3 and 4- rnust operate selectively. Assuming the currentdrawn by load L to be zero, or that load is connected to the particularpoint of the network, then the totalcurrent I flowing from the threesound cables through fuses 1, 2 and 3 into the fault is given by the Thelargest of the three currents I I and I -which may be referred to as I-rnust' not cause the fuse by which it is being carried to blow, whereasI must be sufiiciently high to cause blowing of fuse 4. The ratio I /I*=r (2) selves to the protection of underground cable networks.

The present invention follows in general the above oneof the reasons whyit was not possible, here tofore, to achieve a satisfactory protectionof underground cable networks, V

FIG. 1 presupposes that fuses 1, 2, 3 and 4 are identical,"

fusing {i -dz of" the fuse carrying-the current I current-limiting fusesdesigned to have the smallest postion of the Underwriter and aresubjected to excess currents of different magnitude. On occurrence of ashort-circuit at the point S selective operation of the two fusessubjected to the highest currents I and I will occur if and when thecondition'is met that the sum of the fusing fi 'dt and of the arcing ji-dt of fuse 4 carrying the current I I is smaller than thedisintegration of organic cable insulation a matter of secondaryimportance, then a kind of current-limiting cable protector can beevolved along lines which will be outlined below more in detail. I

Fusible current-limiting protective devices intended to affordshorflcircuit protection as well as overload protec- I tion'shouldinclude multiperforated ribbon fuse link means sible let-through:currents and the highest possible se- 7 lectivity the fusing i -dt andthe arcing fi -dt are substantially equal. Therefore the condition forselective operation can be stated by the following inequalityz whereint,; is the fusing time and r tire ratio defined by 7 Equation 2. HenceIt will be understood that the above calculation has been made assumingthat the currents under interruption are relatively high-short-circuitcurrent-eand the interrupt- V in g times sufficiently short to make ispermissibleto neglect heat exchange phenomena and to consider all ji -dtvalues as being constants.

to .70. Therefore selectivity willbe maintained in the 1 loading, arelatively rapid progressive 'C., e.g. of silver or copper,

of a metal having a fusingpoint in the order of 1000 deg.

and overload-responsive fuselink-severing means having a substantiallylower fusing point. Upon reaching their low fusingpoint thefuselink-severing means sever the current path through the fuse linkmeans. A fuse link in form of a multiperforated silver ribbon having anoverlay of tin-whose fusing point is 232 deg. C.-will be severed by ametallurgical reaction between silver and tin when the latter is heatedup to and beyond its fusing point." If it is intended to provide aprotective fusible device allowing, in case of severe overdisintegrationof organic cable insulation and effecting interruption of the overloadedcircuit only in case the cable becomes a fire hazard, then theoverload-responsive fuse-.link-severing means must become operativeonlyat higher temperatures than the fusing point of tin, preferablyattemperatures In the current-limiting range as long asthe highestpartial current 1 is about 70% but not more than 70% of the totalfault'current I It is noteworthy that this high degree of selectivity iscompatible with the high interrupting capacity inherent incurrent-limiting fuses.

Current-limiting 'fuses'pro gressively decrease the duration of'the fiowof fault currents as the magnitude of available fault currentsincreases. The'total fusing and arcing fi -dz then approaches a' fixedvalue which, is substantially equivalent to a fixed predeterminedtemperature in the range of 300 deg. C. .An overlay of cadmium on aribbon fuse link of silver or copper Will well serve the purpose of anoverload-responsive fuse-link-s'eve'ring means, cadmium having a fusingpoint of 320.9 deg. C.

A'cadmiurn overlay or other overload-responsivelinksevering meansbecoming operative at a temperature in the order of 300 deg. C., if usedinaconventional fuse or a current-limiting fuse does not give rise toany particular problems, but when such an overlay or other link severingmeans which becomes operative in the temperature range 'of'300 deg. C;is used in a fuse that inter-connects two lengths of cable, this givesrise to a .thermal problem. The

reason for this difference in behavior resides in the fact that inconventional fuses and current-limiting fuses there is a steeptemperature gradient from the hot spot to points axially and radiallyspaced from the'hot spot, whereas such a steep temperature gradient, islacking in a pro- 1 .tective device ofthe contemplated nature. Thereforethe rise for a cable of'particular size. This'suggests that.

currentlimiting fuses be applied for the protection of cables providedthatthey can be adapted to meet all requirements of a cable protector.

The Standards for Safety Fuses UL' 198 Seventh Edi- V V Laboratories,Inc., April 1955 (reprinted July 1959,) specify the permissible maximum7 temperature rise At above ambient air in degrees centigrade indicatedinthe table below. 7

. Y At on ferrules Fuse rating in amps. or on blades in O.

Maximum-peak transient temperaturesmay be some-' what higher. I TheNational'Electrical Code 1959 specifies in article 310 thereof maximumoperating temperaturesof cables.

casing of such a device will be subjected to very high temperatures. Ifthe casing is made whollyfor in part of an organic insulating materialthat is allowed to deteriorate progressively in the samefashion as theinsulation of the cable 'is allowed to deteriorate, then the casing maynot be able to withstand the pressure incident to a severe circuitinterruption. It is, therefore, necessary to avoid thermal impairment ofthe casing material 'even though thermal t impairment of thecableinsulation is deemed permissible.

Hence the casing must be made of a mechanically strong heat resistantmaterial, preferably of a silicone-resin-glasscloth laminate. F r

The fusible element or elements of a protective device intended to beconnected into a cable should be able to rapidly'dissipate theheatre'sulting from relatively short overloads. This can be achievedby'providing the protective device with a. pair of terminal elementsincluding blocks of solid metal, e.g. in form of a pair of plugs clos-,ing' the ends of the tubular casing or fuse tube. Such terminalelements are Well known and used in the cur- V rent-limiting fuse artand are shown, for instance, in

1 US. Patent 2,658,974 to Frederick J. Kozacka, Novem- The maximumoperating temperature of moisture] and heat resistant rubber cables hasbeen specifiedto'be 60 C., and the maximum operating temperatureiofsilicone v absestos cable hasbeen specifiedrto be C. Maximumpeaktransient cable temperatures may be somewhat higher. It is consideredsafe practice to allow maximum- C.l75 admitting 'buta moderate.reduction of the useful cable life.. Ifcontinuityof service is,considereda matter of prime importance,'and progressive, destructive Ipeak transient: temperatures of rubber'cables up to 'ber 10, 1953, HighCurrent Carrying Capacity Current- ,Lim'iting Fuses. It ishardly'feasible to spot-weldfusible elements in ribbon-form to terminalelements'in the form of blocks of solid metal as, for instance, term n lelements in the'form of plugs of solid metal closingtheends ofthe'casing or fuse tube. The only feasible or-practical way ofconductivelyconnecting fusible elements inribbon form to terminalelementsin the form ofjblocksof solid metal is by way of softsoldering.It is common practice in current-limiting fuse technology to usetinsolder or similar solder on a tin basis for establishing solder jointsbetween the axially outer ends of ribbon-type fuse link means andterminal elements involving a relatively large mass of copper. .Becauseof the differences between a normal current-limiting fuse anda cableprotector of the kind under consideration, the use of tin solder, or ofa similar solder on a tin basis, is not indicated in the latter case.This will now be explained more in detail in connection ,With FIGS. 2and 3.

Referring now to FIG. 2, this figure illustrates diagrammatically aconventional current-limiting fuse. The current-limiting fuse shown inFIG. 2 comprises a tubular casing 1' of insulating material, apulverulentarc-quenching filler 2' within casing 1', a pair ofcylindrical copper blocks 3 closing the ends of casing 1' and a pair ofblade contacts 4', each integral with one of copper blocks 3.Multiperforated fuse link means 5' of a metal having a fusing point inthe order of 1000 deg. C.silver or copper-conductively interconnectplugs 3'. To this end the axially inner surfaces of plugs 3 may beprovided with grooves into which the axially outer ends of link means 5'may be inserted and which grooves may be filled with tin solder (notshown), or another soft solder which is comparable to tin solder. Eachlink means 5 is provided near the center thereof with an overlay 6' oftin. Overlays '6' are fuse-link-severing means which, upon fusionthereof, sever the-current path through fuse link means 5f by ametallurgical reaction involving the dissolution ofthe high fusing pointbase metal -silver or copperhr the low fusing point overlay metal tinandthe penetration ofmolecules of the low fusing point metal into themolecular structure of the high fusing point metal. The ends of bladecontacts 4 are inserted into a pair of fuse holder contacts 7'.. Thecurve T indicates temperature distribution in a direction longitudinallyof the device when the latter is carrying its rated current. The curve Tindicates the temperature distribution along the axis of the deviceassuming predetermined watt losses occurring therein. It is apparentfromcurve T that the center of. link means 5' has reached the fusingpoint of tin231.8*deg. :C..--at which temperature the link-severingprocess by overlay 6f is initiated. Assuming that the tin overlay: 6'isreplaced by a. link-severing overlay having a higher .fusingpointthantin, eg an overlay of cadmium whosefusing point. is 320.9 deg. C.and further assuming that the wattage dissipated in the structure isincreased to such an extent that the temperature of the hot-spot of thestructure is raised to the fusing point of cadmium, this results in atemperature distribution as represented by. the characteristic T Thetemperature of the blade contacts 4'Whencarrying the rated current isabout 75 deg. C. It is apparent from FIG. 2 that the temperature of thebladecontacts 4 rises but slightly if the load .is increased to such anextent as to result in temperature distribution curves T and T Theparticular temperatureadistribution illustrated in FIG. 2 isprimarilydue to thefact that the temperature of fuse holder contacts 7* remainslow even at currents which are a multiple. of the current rating of the"current-limiting fuse which is being supported bythem. i

In FIG; 3 the same numerals as in FIG. 2, however, with two primesadded, have been applied to indicate like parts asin FIG. 2. Hence FIG.3 does not call for a detailed description in regard to thestructureillustrated therein. Numeral 8" has been applied to indicate the ends ofa cable inserted into tubular cable connector means 4" forming integralparts of copper plugs 3". T indicates the temperature distribution alongthe longitudinal axis of "the device whenit'he latter is carrying itsrated current. T indicates the temperature distribution along thelongitudinal axis of the device when carrying a currentwhich-ifsupplemented by the heating action of cables 8-is' sufiicieritly high tobring the hot-point of the structure to the fusing point of cadmium.When the hot-point of the structureraches the fusing point of cadmiumthetemperature of the insulation of cable 8 is way ferent. This difference.is particularly apparent when comparing the rise of temperature on bladecontacts 4' with the rise of the temperature at the cable connectors 4".As shown in FIG. 2 an increase of the load current from the ratedcurrent to that current which causes blowing by a metallurgical reactionresults in a very small rise in temperature at the fuse holder contacts7'. FIG. 3 shows that a rise of the load current from the rated currentto the current required to cause blowing of the cable protector by ametallurgical reaction results in a very high rise in temperature at thecable connectors 4". This difference is of considerable significance aswill be explained below. V

Referring again to FIG. 3, reference character T has been applied toindicate the temperature distribution along the axis of the device whencarrying a current that is sufliciently low not to result in blowing ofthe fuse even though extending over a long period of time. .Thatparticular current is, however, sufliciently high, to bring the terminalblocks 3" above the temperature of 231.8 deg. C., the fusing temperatureof tin. If tin or a similar low fusing point solder were used toconductively. connect fuse links 5" to terminal blocks 3", this wouldresult in progressive ageing of these solder joints and ultimately information of breaks atthe points where thesefsolder joints are located.Therefore it is not permissible as it is in case of conventionalcurrent-limiting fuses-to use tin solder for connecting the fuse linksto. the terminal blocks. This must be achieved by means of a solderhaving a much higher fusing point than tin, preferably in the order of300 deg. C. Cadmium silver solder serves this purpose well. Such asolder may have a fusing point in the order It will be noted from curveT that the solder joints between fuse links 5" and copper blocks orcopper plugs 3" reach also a temperature in excess of the fusing pointof tin when the current carried by the device is sufficiently high tobringoverlays 6" to the fusing point of cadmium, thus causing blowing ofthe cable protector. During the interrupting process the points of thefuse link where overlay 6" is located will exceed the fusingjpoint ofcadmium and the solder joints between links 5- and plugs 3 may thenexceed the fusing point of the solder of which these solder joints aremade. A softening or even the fusion of the solderjoints between parts 5and 3" incident to blowing has no tendency to impair the sound operationof the device, whereas the gradual deterioration of these solder jointsby currents not sufficiently high to cause fusion of overlays 6" islikely to resultin a dangerous condition and, therefore, must beprecluded by allmeans; l 7 Referring now to FIGS. 4 and 5-,. numeral 21has been applied to indicate a tubular casing of asiliconefresinglass-cloth laminateclosed on the ends thereof by a pairof press-fitted copper plugs 23 held firmly in position-by transversesteel pins 29. Ajpair of tubular cable. connec tors 24 form integralparts of copper plugs 23. Cable connectors 24 may be silver brazed tocopper plugs 23. To this end the axially outer surfaces of plugs 23 areprovided with relatively wide circular grooves 33 into which cableconnectors 24 are inserted. Cables 28 have bare ends inserted into.cable connectors 24 and held therein hy indentations 30 produced by anappropriate hydraulic press tool. The axially inner surfaces 31 ofcopper plugs 23 are pro yided with systems of relatively narrow radialgrooves 32 into which multi-perforated ribbon typefuse' links 25 ofsilver are inserted. The axially outer ends of fuse links 25 aresoldered to plugs 23; by means of joints (not

1. A FUSIBLE PROTECTIVE DEVICE FOR THE PROTECTION OF CABLE NETWORKSCOMPRISING A TUBULAR CASING OF HEAT-RESISTANT INSULATING MATERIAL; APULVERULENT ARC-QUENCHING FILLER WITHIN SAID CASING; A PAIR OF TGERMINALELEMENTS CLOSING THE ENDS OF SAID CASING, EACH OF SAID PAIR OF TERMINALELEMENTS INCLUDING A BLOCK OF SOLID METAL; TUBULAR CABLE CONNECTOR MEANSARRANGED IN COAXIAL RELATION TO SAID CASING FORMING AN INTEGRAL PART OFAT LEAST ONE SAID PAIR OF TERMINAL ELEMENTS; MULTIPERFORATED RIBBON FUSELINK MEANS OF A METAL HAVING A FUSING POINT IN THE ORDER OF 1000 DEG. C.SUBMERSED IN SAID FILLER ADN CONDUCTIVELY INTERCONNECTING SAID PAIR OFTERMINAL ELEMENTS; OVERLOAD-RESPONSIVE FUSE-LINK-SEVERING MEANS OF AMETAL HAVING A FUSING POINT IN THE ORDER OF 300 DEG. C. ARRANGEDADJACENT THE CENTER OF SAID FUSE LINK MEANS FOR SEVERING UPON FUSIONTHEREOF BY A METALLURGICAL REACTION THE CURRENT-PATH THROUGH SAID FUSELINK MEANS, AND SOLDER JOINTS OF A METAL HAVING A FUSING POINT IN THEORDER OF 300 DEG. C. FOR ESTABLISHING CURRENT PATHS OF LOW RESISTANCEBETWEEN SAID PAIR OF TERMINAL ELEMENTS AND SAID FUSE LINK MEANS.